I asked the mechanic at my local MOT test centre if I should get the brake fluid replaced on my car at 44,000 miles. He said it wasn’t necessary with modern brake fluid unless the system has to be bled after, say, fitting a new caliper. Do you agree? C Beck
The importance of brake fluid is often overlooked until the moment it fails. It is hydroscopic, which means it will absorb moisture from the air. If it is in a sealed system, the theory is that it will not absorb moisture because it’s not being exposed to the atmosphere. This is the reason why some people consider replacing the fluid is an unnecessary process.
Many years ago, when I worked on police vehicles, we would regularly replace the brake fluid for another reason. Extensive heating of the brake fluid under harsh braking mean the fluid’s condition and stability will deteriorate, causing any moisture present to boil. While specialist silicone fluids which are hydrophobic will repel moisture in the surrounding air, most brake fluids are glycol-based and will absorb moisture regardless of which DOT rating they have for boiling point.
When replacing brake fluid, it is important to only use fluid from a sealed container. Workshops normally have brake fluid stored in 20-litre sealed bags and dispensed by a tap to prevent the possibility of water/moisture ingress. Silicone fluids are more compressible than glycol fluids and can give a spongy feel to the brake pedal. They should never be mixed with glycol fluids and are not considered a replacement for modern vehicles, but are a suitable alternative for classic cars.